In general, a moving-image delivery system which delivers moving images through a network has been used. Such a moving-image delivery system conforms to a certain standard such as MPEG (Moving Picture Expert Group) or H.264/AVC (Advanced Video Coding), and includes a transmission apparatus which delivers moving images and a reception apparatus which receives moving images.
In the moving-image delivery system, the transmission apparatus encodes moving-image data to be delivered so as to generate a PS (Program Stream) pack from the encoded data. The transmission apparatus transmits a PS including a plurality of PS packs to the reception apparatus through a network. The reception apparatus obtains moving-image data by performing a decoding process or the like on the PS supplied from the transmission apparatus. Thereafter, the reception apparatus outputs the obtained moving-image data to a display, a speaker, or the like. Note that the transmission apparatus may generate a TS (Transport Stream) packet instead of the PS pack and deliver a TS including TS packets to the reception apparatus.
The transmission apparatus and the reception apparatus described above perform a process of matching clock speeds of both the apparatuses with each other using a PCR (Program Clock Reference). This process will be described with reference to FIG. 9. FIG. 9 is a diagram illustrating a clock-speed control process in the related art. As shown in FIG. 9, a transmission apparatus 910 of the related art stores an SCR (System Clock Reference) representing information on a time point when a PS pack is generated in a pack header of the PS pack when generating the PS pack. Then, the transmission apparatus 910 shown in FIG. 9 generates a PS pack 91P and transmits the generated PS pack 91P to a reception apparatus 920. Furthermore, after a period of time t11 is elapsed after the PS pack 91P is generated, the transmission apparatus 910 generates a PS pack 92P and transmits the PS pack 92P to the reception apparatus 920. After a period of time t12 is elapsed after the PS pack 92P is generated, the transmission apparatus 910 generates a PS pack 93P and transmits the PS pack 93P to the reception apparatus 920.
Thereafter, the reception apparatus 920 shown in FIG. 9 generates a PCR in accordance with the SCR included in the PS pack 91P when receiving the PS pack 91P. It is assumed that the reception apparatus 920 generates a PCR having a PCR value of P1. In this case, the reception apparatus 920 sets the PCR value of P1 as an STC (System Time Clock) of the reception apparatus 920. Subsequently, it is assumed that the reception apparatus 920 received the PS pack 92P after a period of time t21 is elapsed after the PS pack 91P is received. In this case, the reception apparatus 920 generates a PCR having a PCR value of P2 in accordance with an SCR included in the PS pack 92P.
Then, the reception apparatus 920 compares the PCR value of P2 with the STC value of the reception apparatus 920. Since the PCR value of P2 represents information on a time point when the transmission apparatus 910 generates the PS pack 92P, the PCR value of P2 is obtained by adding the PCR value of P1 to the period of time t11 (P1+t11). Furthermore, since the STC value of the reception apparatus 920 represents information on a time point when the PCR is generated in accordance with the PS pack 92P, the STC value is obtained by adding the PCR value of P1 to the period of time t21 (P1+t21). Specifically, the reception apparatus 920 compares the elapsed period of time t11 of the transmission apparatus 910 with the elapsed period of time t21 of the reception apparatus 920. As a result of the comparison, when the PCR value of P2 is larger than the STC value, the reception apparatus 920 determines that a clock speed of the reception apparatus 920 is lower than a reference clock speed of the transmission apparatus 910 and makes the clock speed of the reception apparatus 920 be increased. On the other hand, when the PCR value of P2 is smaller than the STC value, the reception apparatus 920 determines that the clock speed of the reception apparatus 920 is higher than the reference clock speed of the transmission apparatus 910 and makes the clock speed of the reception apparatus 920 be reduced.
Furthermore, when receiving the PS pack 93P after the period of time t22 is elapsed after the PS pack 92P is received, the reception apparatus 920 shown in FIG. 9 generates a PCR having a PCR value of P3 in accordance with an SCR included in the PS pack 93P. Then, the reception apparatus 920 compares the PCR value of P3 with the STC value of the reception apparatus 920 so as to control the clock speed of the reception apparatus 920.
This method is disclosed in Japanese Laid-open Patent Publication No. 9-307865, for example.
Furthermore, in the related art, a packet arrival time may be shifted due to a network jitter or the like. When a large shift is generated, a difference between a generation time of the transmission apparatus and an arrival time of the reception apparatus becomes considerably large, and accordingly, time is not reliably controlled. To address this problem, as a countermeasure, a threshold value is set for a result of a difference obtained from a comparison between the PCR value and the STC value. In this countermeasure, when the result is larger than the threshold value, the PCR is not used.
Such a countermeasure is disclosed in Japanese Laid-open Patent Publication No. 2003-218842, for example.